This invention relates to method and means for forming a more even web of staple fibers of randomized orientation suitable for nonwoven end uses. The present invention employs centrifugal, dynamic web forming. A method and means therefor known in the art are as disclosed in U.S. Pat. No. 4,279,060 of July 21, 1981. Generally, there one uses a carding action to disentangle the staple fibers of fibrous flocks and form a web of a layer of such fibers oriented such that their axes extend more parallel one another than theretofore and in the direction of fiber flowpath in the process machinery. The resultant web removed from the carding device is subjected to an accumulator, or bunching or jamming or damming action to increase its depth to provide a more dense web of greater than one-fiber thickness, and thereafter is subjected to centrifugal aerodynamic forces, so called free web formation, and then doffed from the apparatus. The doffed web then is suitable for producing or for use as nonwoven fabrics. Depending upon the degree of bunching and after-treatment of the webs produced, they are suitable for providing so-called light weight (from 8 to 25 grams of web per square meter of surface area), medium weight (from 25 to 75 grams per square meter) or heavy weight (from 70 to 250 grams per square meter) nonwoven fabrics.
The means of U.S. Pat. No. 4,279,060 is particularly instructive in understanding the foregoing general method. There one employs the well known carding cylinder and doffer; however, interposed therebetween is the cenrifugal, aerodynamic web former. It comprises an intermediate toothed cylinder and a special guide surface adjacent thereto. The cylinder is fitted with a toothed surface wherein the protruding teeth have a front rake thereof set at an angle of from zero to just a few angular degrees, whereby the extention of the front rake of the toothing theoretically passes through the center point of the circular path formed by the roller or cylinder surface bearing the toothing. Coacting with the toothing is a guide plate set along a portion of the circumference of the cylinder but is spaced somewhat away therefrom along the flowpath of material carried by the toothing such that the spacing progressively diminishes downstream so that, in example, at the entry to the space between the teeth and the guide plate's adjacent surface the spacing may be 3 to 6 millimeters and then diminish to a spacing of only one millimeter at the region of the exiting of the material from the space. This produces what is now known as the accumulating, bunching, jamming or damming action. Then when the bunched fibrous layer or web leaves the confined space above defined and the teeth of the intermediate roller cylinder, by what is believed to be inertia and centrifugal forces, it is flung a short distance, there said to be in example 8 to 12 millimeters, through what is called a free zone passing therethrough in aerodynamic flight until it encounters the teeth of the doffer roll. Evidentally, during such aerodynamic flight, the web is freed from the jamming effect and evens out to form the random non-woven web of staple fibers useful for forming or as a non-woven fabric.
The problem addressed by the aforesaid U.S. Pat. No. 4,279,060 is one of achieving a randomization of carded fibrous web in a relatively simple manner using, for the most part, carding and doffing elements well known in the art and easily available, without the need for expensive auxiliary machines, such as crosslappers. In this, attempts to attain greater and yet greater isotropy respecting the axes of fibers in the family of planes of the web, such as a greater equivalence of tear strengths and processabilities or the like in the directions transverse to the web length as in the direction of the web length, were sought.
While the approach employed in U.S. Pat. No. 4,279,060 was largely successful to provide an economical way to obtain randomization of the fibers axes, another factor arose which demanded consideration. It appeared that air flows and the agitative regime thereof, in the spacial regions where the parallelized staple fibers move into the web former slot between the teeth of the toothed or carding cylinder and the guide surface which defines a progressively diminishing depth thereof, have a profound affect upon the uniformity of the web produced in depth as well as randomness of orientation of the axes of the constituent fibers.
Depending upon the stock being processed, the degree of carding prior to its being subjected to randomizing, aerodynamic web forming, the surface speeds of the toothed cylinder and the doffer cylinder from which the fibers being randomized are flung into aerodynamic free flight and onto which they are caught, the respective surfaces of the moving toothed cylinder and the fixed slot forming plate or other member which define the jamming slot of the web former, as well as the direction, volume and speed of flow of the sundry air currents being generated thereat, one notes that there is complexity regarding prescribing randomness of orientation of the directions of the axes of the constituent fibers of the web produced and also, as mentioned, regarding the uniformity of web transversely as well as from length portion to length portion therealong being produced.
While many of these factors may be prescribably varied to desired values so that desired results may be obtained, at least one factor has heretofore eluded the art's attempts in this regard, that of prescribing and controlling the air flows generated and the resultant agitative regime produced thereby in the region of the web forming slot. It is to this vexsome problem that the present invention is addressed. In this one notes that the toothed slot-defining cylinder is operated at unusually high surface speeds in necessity of providing the high production rates demanded by commercial economics; further, that in so doing high concentrations of dust and micro-dust are liberated and which must through some means be contained for removal, as health standards and laws require, yet further compounding the problem here addressed.